1. Field of the Invention
This invention relates to a system for handling pulse encoded signals to minimize the effect of dropouts of such signals when they are transmitted along electrically parallel paths.
2. The Prior Art
It is common practice to encode pulse signals to correspond to the amplitude of an analog signal measured, or sampled, at specific time intervals. A typical pulse code modulation system involves producing a multi-digit pulse signal for each measurement of the analog signal. The encoding system uses a certain number of pulses for each code group, and the pulses are typically based on the binary value that corresponds to the analog amplitude. For example, a four-digit pulse signal could be used to encode an analog signal having a value between 0 and 15 volts by assigning to the pulses certain amplitude values separated by 1 volt of the analog signal amplitude. Thus, in the system that provided time for transmitting the four pulses, and with each of the pulses having either a value identified as 0 or 1, a group of pulses 1010 would correspond to an analog voltage of 10 volts.
In transmitting such pulse signals it has been common to separate the pulses of each group according to the weighting factor of each pulse. In the case of a four-digit pulse signal, four electrically parallel signal paths would be required. One path would transmit the first digit signal, which would be either a 1 or a 0, a second path would transmit the second digit, a third path the third digit, and a fourth path the fourth digit. In demodulating the four-digit pulse signal to reproduce the analog signal having a value between 0 and 15 volts, the fourth digit pulse signal, which is also called the fourth bit, would add one volt to the analog signal obtained after demodulating the first three digits. Thus, the fourth digit signal is spoken of as having a weighting factor of one and is called the least significant bit. Correspondingly, the third digit signal, or third significant bit, would affect the analog value of the demodulated signal by 2 volts, or twice as much as the least significant bit signal. The second digit pulse signal, or second significant bit, would affect the value of the analog signal by twice as much as the third significant bit, or 4 volts in the example being considered, and the first digit pulse signal, which is also the most significant bit, would affect the value of the demodulated analog signal by twice as much as the second significant bit, or 8 volts in the example.
Analog signals that have been encoded in this pulse form can be transmitted in parallel paths so that all of the digits will arrive at the other end of the path simultaneously. In order to be translated back into an analog signal form, the output pulse signals from the transmission paths must be decoded in the proper sequence and given the proper weighting factor. To refer again to the example, a pulse signal 0001 would be decoded into an analog signal of 1 volt and a pulse signal 1111 would be decoded into an analog signal 15 volts. Signals corresponding to binary numbers 0001 and 1111 would be decoded as analog signals having values between 1 and 15 volts.
It unfortunately happens that sometimes one bit of a multibit, or multi-digit, pulse signal is lost in the transmission path. For example, the transmission path may consist of magnetic tape, and a small particle of dust may interfere with the recording of one of the bits of the signal. Another possibility is that there may be a scratch or some other defect on the magnetic surface of the tape that causes the magnetic material to be missing at a certain place where one of the bits should be recorded. If the missing bit happens to be the least significant bit, the resulting analog signal will not be changed very much (unless the loss of a single bit offsets the entire decoding system) but if the bit that is lost is the one that has the greatest weighting factor and is, therefore, known as the most significant bit, it will at least change the amplitude of the decoded signal by a large amount.
The loss of one bit of a multi-digital signal has heretofore been compensated for by arbitrarily assuming that the value of the signal did not change between the preceding group of signals and the group in which there was a missing bit or between the group in which there was a missing bit and the succeeding group of signals. By replacing the missing bit with the bit of the same weighting factor from either the succeeding or following group of signals, based on the assumption that the value of the amplitude signal will not vary much from one sample to the next, a reconstituted digital signal can be produced that is likely to be of the correct value or at least not greatly different from the correct value. However, it sometimes happens that two or more bits in succession on a given transmission path will be dropped out, and then it becomes difficult to reestablish a reasonable value for the missing bits.
In transmitting pulse coded signals it is frequently desirable that they not be required to maintain a base value that corresponds, for example, to the 0 level and with all of the pulses that represent 1's differing from the 0 value in one polarity. A signal that is arranged so that 0 signals have one polarity and 1 signals have the opposite polarity may be preferred. In the case of such signals, the voltage swings back and forth between the two values and does not return to 0. Such signals are, therefore, referred to as non-return-to-zero or NRZ signals. The loss of one bit of an NRZ signal may cause the voltage in the transmission path to return to zero, which is an ambiguous condition since it represents neither a 1 nor a 0. This makes it more difficult to interpolate for a missing bit or a missing succession of bits of the same weighting factor.
Since the encoding is based on time, it is necessary to transmit timing information along with the multi-digit pulse signal. The timing information can be sent on a separate parallel signal path, but this is undesirable because it takes one of the available signal paths that might otherwise be used for more useful information. Accordingly, signals that have been encoded as pulses are sometimes subjected to further encoding by what is known as dynamic modulation (DM) or phase encoding (PE). Such signals include timing information without the necessity of providing an extra channel. However, it is necessary to adhere to the requirements of such signals in replacing dropout signal bits.
Accordingly, it is one object of the present invention to provide a singal handling system for minimizing dropout effects in multi-digit signals transmitted along parallel signal paths.
Another object is to provide for minimizing errors that might be caused by the dropout of bit signals of NRZ pulse code modulation signals.
A still further object of the present invention is to provide a handling system to minimize errors caused by dropout of bit signals that include timing information along with the signal information.
Still further objects of the present invention will be apparent from the following specification together with the drawings.